/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.io.ObjectStreamField;

import java.util.Random;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.atomic.AtomicLong;

/**

 * A random number generator isolated to the current thread.  Like the

 * global {@link java.util.Random} generator used by the {@link

 * java.lang.Math} class, a {@code ThreadLocalRandom} is initialized

 * with an internally generated seed that may not otherwise be

 * modified. When applicable, use of {@code ThreadLocalRandom} rather

 * than shared {@code Random} objects in concurrent programs will

 * typically encounter much less overhead and contention.  Use of

 * {@code ThreadLocalRandom} is particularly appropriate when multiple

 * tasks (for example, each a {@link ForkJoinTask}) use random numbers

 * in parallel in thread pools.

 *

 * <p>Usages of this class should typically be of the form:

 * {@code ThreadLocalRandom.current().nextX(...)} (where

 * {@code X} is {@code Int}, {@code Long}, etc).

 * When all usages are of this form, it is never possible to

 * accidently share a {@code ThreadLocalRandom} across multiple threads.

 *

 * <p>This class also provides additional commonly used bounded random

 * generation methods.

 *

 * @since 1.7

 * @author Doug Lea

 */

public class ThreadLocalRandom extends Random {

    /*

     * This class implements the java.util.Random API (and subclasses

     * Random) using a single static instance that accesses random

     * number state held in class Thread (primarily, field

     * threadLocalRandomSeed). In doing so, it also provides a home

     * for managing package-private utilities that rely on exactly the

     * same state as needed to maintain the ThreadLocalRandom

     * instances. We leverage the need for an initialization flag

     * field to also use it as a "probe" -- a self-adjusting thread

     * hash used for contention avoidance, as well as a secondary

     * simpler (xorShift) random seed that is conservatively used to

     * avoid otherwise surprising users by hijacking the

     * ThreadLocalRandom sequence.  The dual use is a marriage of

     * convenience, but is a simple and efficient way of reducing

     * application-level overhead and footprint of most concurrent

     * programs.

     *

     * Because this class is in a different package than class Thread,

     * field access methods use Unsafe to bypass access control rules.

     * The base functionality of Random methods is conveniently

     * isolated in method next(bits), that just reads and writes the

     * Thread field rather than its own field.  However, to conform to

     * the requirements of the Random superclass constructor, the

     * common static ThreadLocalRandom maintains an "initialized"

     * field for the sake of rejecting user calls to setSeed while

     * still allowing a call from constructor.  Note that

     * serialization is completely unnecessary because there is only a

     * static singleton.  But we generate a serial form containing

     * "rnd" and "initialized" fields to ensure compatibility across

     * versions.

     *

     * Per-thread initialization is similar to that in the no-arg

     * Random constructor, but we avoid correlation among not only

     * initial seeds of those created in different threads, but also

     * those created using class Random itself; while at the same time

     * not changing any statistical properties.  So we use the same

     * underlying multiplicative sequence, but start the sequence far

     * away from the base version, and then merge (xor) current time

     * and per-thread probe bits to generate initial values.

     *

     * The nextLocalGaussian ThreadLocal supports the very rarely used

     * nextGaussian method by providing a holder for the second of a

     * pair of them. As is true for the base class version of this

     * method, this time/space tradeoff is probably never worthwhile,

     * but we provide identical statistical properties.

     */

    // same constants as Random, but must be redeclared because private

    private static final long multiplier = 0x5DEECE66DL;

    private static final long addend = 0xBL;

    private static final long mask = (1L << 48) - 1;

    private static final int PROBE_INCREMENT = 0x61c88647;

    /** Generates the basis for per-thread initial seed values */

    private static final AtomicLong seedGenerator =

        new AtomicLong(1269533684904616924L);

    /** Generates per-thread initialization/probe field */

    private static final AtomicInteger probeGenerator =

        new AtomicInteger(0xe80f8647);

    /** Rarely-used holder for the second of a pair of Gaussians */

    private static final ThreadLocal<Double> nextLocalGaussian =

        new ThreadLocal<Double>();

    /**

     * Field used only during singleton initialization.

     * True when constructor completes.

     */

    boolean initialized;

    /** Constructor used only for static singleton */

    private ThreadLocalRandom() {

        initialized = true; // false during super() call

    }

    /** The common ThreadLocalRandom */

    static final ThreadLocalRandom instance = new ThreadLocalRandom();

    /**

     * Initialize Thread fields for the current thread.  Called only

     * when Thread.threadLocalRandomProbe is zero, indicating that a

     * thread local seed value needs to be generated. Note that even

     * though the initialization is purely thread-local, we need to

     * rely on (static) atomic generators to initialize the values.

     */

    static final void localInit() {

        int p = probeGenerator.getAndAdd(PROBE_INCREMENT);

        int probe = (p == 0) ? 1 : p; // skip 0

        long current, next;

        do { // same sequence as j.u.Random but different initial value

            current = seedGenerator.get();

            next = current * 181783497276652981L;

        } while (!seedGenerator.compareAndSet(current, next));

        long r = next ^ ((long)probe << 32) ^ System.nanoTime();

        Thread t = Thread.currentThread();

        UNSAFE.putLong(t, SEED, r);

        UNSAFE.putInt(t, PROBE, probe);

    }

    /**

     * Returns the current thread's {@code ThreadLocalRandom}.

     *

     * @return the current thread's {@code ThreadLocalRandom}

     */

    public static ThreadLocalRandom current() {

        if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)

            localInit();

        return instance;

    }

    /**

     * Throws {@code UnsupportedOperationException}.  Setting seeds in

     * this generator is not supported.

     *

     * @throws UnsupportedOperationException always

     */

    public void setSeed(long seed) {

        // only allow call from super() constructor

        if (initialized)

            throw new UnsupportedOperationException();

    }

    protected int next(int bits) {

        Thread t; long r; // read and update per-thread seed

        UNSAFE.putLong

            (t = Thread.currentThread(), SEED,

             r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);

        return (int) (r >>> (48-bits));

    }

    /**

     * Returns a pseudorandom, uniformly distributed value between the

     * given least value (inclusive) and bound (exclusive).

     *

     * @param least the least value returned

     * @param bound the upper bound (exclusive)

     * @throws IllegalArgumentException if least greater than or equal

     * to bound

     * @return the next value

     */

    public int nextInt(int least, int bound) {

        if (least >= bound)

            throw new IllegalArgumentException();

        return nextInt(bound - least) + least;

    }

    /**

     * Returns a pseudorandom, uniformly distributed value

     * between 0 (inclusive) and the specified value (exclusive).

     *

     * @param n the bound on the random number to be returned.  Must be

     *        positive.

     * @return the next value

     * @throws IllegalArgumentException if n is not positive

     */

    public long nextLong(long n) {

        if (n <= 0)

            throw new IllegalArgumentException("n must be positive");

        // Divide n by two until small enough for nextInt. On each

        // iteration (at most 31 of them but usually much less),

        // randomly choose both whether to include high bit in result

        // (offset) and whether to continue with the lower vs upper

        // half (which makes a difference only if odd).

        long offset = 0;

        while (n >= Integer.MAX_VALUE) {

            int bits = next(2);

            long half = n >>> 1;

            long nextn = ((bits & 2) == 0) ? half : n - half;

            if ((bits & 1) == 0)

                offset += n - nextn;

            n = nextn;

        }

        return offset + nextInt((int) n);

    }

    /**

     * Returns a pseudorandom, uniformly distributed value between the

     * given least value (inclusive) and bound (exclusive).

     *

     * @param least the least value returned

     * @param bound the upper bound (exclusive)

     * @return the next value

     * @throws IllegalArgumentException if least greater than or equal

     * to bound

     */

    public long nextLong(long least, long bound) {

        if (least >= bound)

            throw new IllegalArgumentException();

        return nextLong(bound - least) + least;

    }

    /**

     * Returns a pseudorandom, uniformly distributed {@code double} value

     * between 0 (inclusive) and the specified value (exclusive).

     *

     * @param n the bound on the random number to be returned.  Must be

     *        positive.

     * @return the next value

     * @throws IllegalArgumentException if n is not positive

     */

    public double nextDouble(double n) {

        if (n <= 0)

            throw new IllegalArgumentException("n must be positive");

        return nextDouble() * n;

    }

    /**

     * Returns a pseudorandom, uniformly distributed value between the

     * given least value (inclusive) and bound (exclusive).

     *

     * @param least the least value returned

     * @param bound the upper bound (exclusive)

     * @return the next value

     * @throws IllegalArgumentException if least greater than or equal

     * to bound

     */

    public double nextDouble(double least, double bound) {

        if (least >= bound)

            throw new IllegalArgumentException();

        return nextDouble() * (bound - least) + least;

    }

    public double nextGaussian() {

        // Use nextLocalGaussian instead of nextGaussian field

        Double d = nextLocalGaussian.get();

        if (d != null) {

            nextLocalGaussian.set(null);

            return d.doubleValue();

        }

        double v1, v2, s;

        do {

            v1 = 2 * nextDouble() - 1; // between -1 and 1

            v2 = 2 * nextDouble() - 1; // between -1 and 1

            s = v1 * v1 + v2 * v2;

        } while (s >= 1 || s == 0);

        double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);

        nextLocalGaussian.set(new Double(v2 * multiplier));

        return v1 * multiplier;

    }

    // Within-package utilities

    /*

     * Descriptions of the usages of the methods below can be found in

     * the classes that use them. Briefly, a thread's "probe" value is

     * a non-zero hash code that (probably) does not collide with

     * other existing threads with respect to any power of two

     * collision space. When it does collide, it is pseudo-randomly

     * adjusted (using a Marsaglia XorShift). The nextSecondarySeed

     * method is used in the same contexts as ThreadLocalRandom, but

     * only for transient usages such as random adaptive spin/block

     * sequences for which a cheap RNG suffices and for which it could

     * in principle disrupt user-visible statistical properties of the

     * main ThreadLocalRandom if we were to use it.

     *

     * Note: Because of package-protection issues, versions of some

     * these methods also appear in some subpackage classes.

     */

    /**

     * Returns the probe value for the current thread without forcing

     * initialization. Note that invoking ThreadLocalRandom.current()

     * can be used to force initialization on zero return.

     */

    static final int getProbe() {

        return UNSAFE.getInt(Thread.currentThread(), PROBE);

    }

    /**

     * Pseudo-randomly advances and records the given probe value for the

     * given thread.

     */

    static final int advanceProbe(int probe) {

        probe ^= probe << 13;   // xorshift

        probe ^= probe >>> 17;

        probe ^= probe << 5;

        UNSAFE.putInt(Thread.currentThread(), PROBE, probe);

        return probe;

    }

    /**

     * Returns the pseudo-randomly initialized or updated secondary seed.

     */

    static final int nextSecondarySeed() {

        int r;

        Thread t = Thread.currentThread();

        if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {

            r ^= r << 13;   // xorshift

            r ^= r >>> 17;

            r ^= r << 5;

        }

        else {

            localInit();

            if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)

                r = 1; // avoid zero

        }

        UNSAFE.putInt(t, SECONDARY, r);

        return r;

    }

    // Serialization support

    private static final long serialVersionUID = -5851777807851030925L;

    /**

     * @serialField rnd long

     *              seed for random computations

     * @serialField initialized boolean

     *              always true

     */

    private static final ObjectStreamField[] serialPersistentFields = {

            new ObjectStreamField("rnd", long.class),

            new ObjectStreamField("initialized", boolean.class)

    };

    /**

     * Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).

     */

    private void writeObject(java.io.ObjectOutputStream out)

        throws java.io.IOException {

        java.io.ObjectOutputStream.PutField fields = out.putFields();

        fields.put("rnd", UNSAFE.getLong(Thread.currentThread(), SEED));

        fields.put("initialized", true);

        out.writeFields();

    }

    /**

     * Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.

     */

    private Object readResolve() {

        return current();

    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;

    private static final long SEED;

    private static final long PROBE;

    private static final long SECONDARY;

    static {

        try {

            UNSAFE = sun.misc.Unsafe.getUnsafe();

            Class<?> tk = Thread.class;

            SEED = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomSeed"));

            PROBE = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomProbe"));

            SECONDARY = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomSecondarySeed"));

        } catch (Exception e) {

            throw new Error(e);

        }

    }

}